This article presents a theoretical framework for the evolution of the light-element nuclides in the Galactic disk. By understanding this evolution correctly, we can reliably obtain the primordial abundances of the nuclides D, 4He, and 7Li. We use two key assumptions, those of (1) infall of metal-poor gas to the disk at an increasing rate and (2) destruction, as well as production (except for D), of fragile nuclides in hot, relatively dense supergiant envelopes. Light nuclides are accelerated by supernova shocks, and many are confined to hot interstellar zones by magnetic fields. Their repeated passage through the hot envelopes causes depletion, which peaked during the main star-forming phase of Galaxy evolution around z ≈ 1, as measured from the Hubble Deep Field. This mechanism has dominated stellar depletion in reducing the D/H abundance from its primordial value of ≃2 × 10-4 to its solar system value of ≃2.5 × 10-5 and subsequently to the current interstellar medium value of 1.5 × 10-5. The model accounts well for the solar system and the current ratios of 7Li/6Li and 11B/10B. It fits extremely well a standard big bang nucleosynthesis model with baryon density ≃0.05.